[MLIR][Presburger] enable copy assignment operator for Simplex

[webmiche]

Summary

This patch removes the const from usingBigM to enable the implicit copy assignment operator for Simplex.

mlir/include/mlir/Analysis/Presburger/Simplex.h

protected:
   /// Undo the operation represented by the log entry.
   void undo(UndoLogEntry entry);
 
   /// Return the number of fixed columns, as described in the constructor above,
   /// this is the number of columns beyond those for the variables in var.
   unsigned getNumFixedCols() const { return usingBigM ? 3u : 2u; }
 
   /// Stores whether or not a big M column is present in the tableau.
-  const bool usingBigM;
+  bool usingBigM;
 
   /// The number of rows in the tableau.
   unsigned nRow;
 
   /// The number of columns in the tableau, including the common denominator
   /// and the constant column.
   unsigned nCol;